The application of thin coatings, e.g., less than about 10 microns, has become an increasingly important step in the manufacturing of various products such as, for example: flat panel displays such as used in lap top computers, high definition television and computer cathode ray tubes; optical devices such as lenses, color filters and mirrors; hybrid circuit boards, silicon wafers and germanium wafers.
As the thickness of such coatings decreases, e.g., to about 1 micron or less, it is necessary for the surfaces of the objects being coated to be extremely clean in order to insure minimal defects in the coatings. For example, some active display panels have as many as 10 to 15 layers of coatings, each having thicknesses of from about 500 angstroms to 5 microns. Some wafers have conductive line spacings on the order of about 0.5 micron. Accordingly, particles even as small as about 1 micron or less can interfere and cause defects in these extremely thin coatings.
Ultrasonic transducers are often used to introduce vibrations into cleaning solutions to enhance the removal of particles from the surfaces of objects. Ultrasonic transducers vibrate at frequencies of from about 10 to 40 kilohertz ("KHz"). While ultrasonic cleaning is effective to move relatively large particles, it is virtually ineffective in removing particles having a particle size of about 1 micron or less.
Megasonic cleaning is a method similar to ultrasonic cleaning, but uses vibration frequencies of approximately 800 KHz to 1.8 megahertz ("MHz"). Megasonic cleaning can be highly effective for removing particles having a particle size of about 1 micron or less.
Ultrasonic and megasonic cleaning methods usually involve immersing the objects in a bath of liquid cleaning fluid and introducing the ultrasonic or megasonic vibrations through the cleaning fluid to dislodge particles attached to the surface of the objects. Such cleaning methods usually operate in a batch mode, thereby requiring subsequent handling of the objects, either by robots or humans, after cleaning and prior to the next process step, e.g., coating. Accordingly, small particles can readily attach to the surfaces during such handling.
One particularly useful method for applying thin coatings to objects is described in U.S. Pat. No. 4,370,376, issued Jan. 25th, 1983. At Column 1, lines 40 to 57, the patentees disclose that:
"According to the present invention, meniscus coating of an object such as a substrate is accomplished by flowing a coating material through a permeable and sloping surface, so as to develop a downward laminar flow of coating material on the outside of the sloping surface. The object, having a surface to be coated, is advanced tangentially to the downward laminar flow of coating material, such that the surface to be coated intersects the laminar flow of coating material at the apex of the sloping, permeable surface menisci of flowing coating material are supported both at the leading edge and the trailing edge of coating material in contact with the surface to be coated. The uniform disengagement and drainage of deposited excess coating material from the coated surface are ensured by uniform menisci and the constant downward laminar flow of coating material on the outside of the sloping surface." PA1 "The cleaning processes may include solvent washing with brushes, ultrasonic scrubbing and/or other mechanical scrubbing methods. The substrate may then be rinsed with high purity water and/or solvent prior to solvent drying and removal. A porous cylindrical applicator may be utilized, also, to clean and rinse the substrate with solvents. Solvent drying may include evaporative operations via controlled gas flow and/or vacuum processes."
The patentees further disclose that surface of the object may be cleaned prior to coating. At Column 6, lines 38 to 46, the patentees disclose that:
However, although such cleaning methods are typically effective to remove relatively large particles, e.g., greater than about 1 micron, they are often ineffective to remove particles having particle sizes of less than about 1 micron.
Accordingly, methods and apparatus are desired for cleaning the surfaces of objects to remove small particles, e.g., less than about 1 micron. In addition, methods and apparatus for cleaning such objects are desired which can be utilized immediately prior to subsequent processing steps which are sensitive to the presence of the particles.